In my Migrating to IIS7 post, I highlighted that building REST frameworks that do meaningful things with Urls is hard on microsoft's platforms.
I encourage you to go and read the comments left by Mike Volodarsky on the issue. He's a Program Manager at Microsoft. Having people from the team building IIS7 responding to people like me highlighting issues shows how some teams really engage proactively with their customers. Kudos!
The other team at Microsoft that understand REST a bit (and IMO the most out of all the teams working on this), is the Astoria team. And of course, because they do funky stuff with their URLs, they hit the same issues as me. So here's a link on how to deactivate some of the checks asp.net does. Nasty registry editing...
I really wish asp.net could recognize when an IHttpHandler does not want validation and let it handle the request, its validation and etc.